Connection by balls made of materials having a low melting point is widely used to connect two electronic circuits and thus forming a composite or hybrid circuit. There are balls intended for a purely mechanical connection of circuits and other balls which connect the conductive tracks of the two circuits and above all providing an electric connection. However, generally speaking, the balls simultaneously provide both the mechanical and electrical connection functions, as described in the patent application WO 7/01509.
A certain number of hybrid circuits are subjected to significant thermic amplitudes during the time they are functioning. When the materials of the substrate of the two electronic circuits have different coefficients of expansion, the balls are subjected to shearing stresses which end up by breaking them directly or by virtue of a fatigue phenomenon. The stresses are normally greater at the periphery of the two electronic circuits as the displacements due to the expansions are more marked. When a ball is broken, it may obviously no longer connect the two electronic circuits, the functioning of the latter being compromised if an electric connection ball is used.
A widely used technique to prolong the life of the hybrid circuit consists of acting on the shape of the connection balls. When the material intended to constitute a ball is melted so as to moisten the electronic circuits and form a weld once it has resolidified, it takes a shape which depends on its volume, the spacing of the electronic circuits and the wettable surfaces of the two circuits to which this material adheres and on which it is spread in a liquid state. Thus, it is possible to correct the action of the surface tension forces which would provide spherical balls so as to obtain small column-shaped balls which are more or less slender, bulging or, on the other hand, narrowed at the center, as described in the patent application JP-A-62 15 0837 (Patents abstracts of Japan, vol. 11, No 384).
The small column-shaped slender or slightly bulging balls, that is not having a spherical shape, are in fact only subjected to concentrations of moderate stresses. Thus, the problems of fatigue are reduced but without completely eliminating them.
Another problem described in the article by Norio Matsui and published in IEEE Transactions on Components, hybrids and Manufacturing Technology (vol. CHMT 12, No 4, December 1987 p.566-570) consists of vertically juxtaposing the balls so as to obtain columns in which the stresses would be reduced. In this case, the result obtained being equivalent to the previous one, is not satisfactory and requires the use of extremely heavy equipment and involves a large number of delicate operations.